Temp signature ban is just short-term solution, and they will come back with Cryptotalk or other campaigns when their signature temporary bans expired.
Generally I don't agree with signature ban of every one banned from Cryptotalk campaign. But I think temporary signature ban can be effective. When a user is temporary banned, at least he is warned about the quality of his posts and he can be permanently banned, if the behavior doesn't change.
First offence: 7 days
Second offence: 14 days
Third offence: 30 days
Fourth: Permanent ban